The present invention relates to the field of electronic circuits, and, more particularly, to low drop-out (LDO) type linear voltage regulators, namely low serial voltage drop-out regulators.
Low drop-out (LDO) type linear voltage regulators, such as low serial voltage drop-out regulators, are used in a variety of applications. In particular, these regulators may be used in mobile telephones to deliver a regulated voltage from a battery power supply voltage to radio transmission/reception circuits.
By way of example, a standard linear regulator 10 is illustrated in FIG. 1. An output of the regulator 10 delivers a regulated voltage Vout to a load Z. The load Z represents, for example, several radio circuits present in a mobile telephone. The regulator 10 is powered by a voltage Vbat delivered by a battery 1 and comprises a differential amplifier 2 whose output drives the gate G of a P-channel metal oxide semiconductor (PMOS) regulation transistor 3. The output stage of the amplifier 2 has an internal resistance Rg (or gate resistance), shown in dashes, that determines the gain of the amplifier 2 and the maximum current that it can deliver at the output.
The transistor 3 receives the voltage Vbat at its source S. Its drain D, connected to the output of the regulator 10, is connected to the anode of a capacitor Cst for filtering and stabilizing the voltage Vout. This capacitor Cst is parallel-connected with the load Z. The amplifier 2 receives a reference voltage Vref at its negative input and a feedback voltage Vfb at its positive output. The voltage Vfb is, for example, a fraction of the voltage Vout provided to the input of the amplifier 2 by a divider bridge including two resistors R1, R2.
Operation of a regulator of this kind, which is well known to those skilled in the art, includes modulating the gate voltage Vg of the transistor 3 using the amplifier 2. This is done as a function of the difference between the voltage Vfb and the reference voltage Vref. When the voltage Vg is substantially smaller than Vbatxe2x88x92Vtp, the transistor 3 is on because its gate-source voltage Vgs is substantially higher than the threshold voltage Vtp. When the voltage Vg is higher than Vbatxe2x88x92Vtp, the transistor 3 is off. In a stabilized state, the voltage Vout is regulated in the neighborhood of its nominal valve Voutnom, which is equal to [(R1+R2)Vref/R2].
In an application such as supplying power to the radio circuits of a mobile telephone, it is important that the amplifier 2 consume as little electricity as possible to maintain the charge stored in the battery. To this end, the gate resistance Rg of the output stage of the amplifier 2 should be chosen so that it has a high value (e.g., 100 Kxcexa9) to limit the maximum current flowing in the output stage to the high state.
Furthermore, the regulation transistor 3 must have a low serial resistance RdsON in the on state (drain-source resistance) so that it can deliver high current without any prohibitive voltage drop-out at its terminals. Thus, the transistor 3 conventionally has a high gate width-to-length ratio. For example, the transistor 3 may have a gate width W of 2xc3x97105 micrometers for a gate length L of 0.6 micrometers, giving a W/L ratio in the range of 3xc3x97105 micrometers and a very great transistor width. Due to its size and its high W/L ratio, the transistor 3 also has a high gate capacitance Cg (shown in dashes in FIG. 1), in the range of 100 to 200 picofarads.
These various characteristics are indispensable for obtaining a regulator with low consumption and low serial voltage drop-out. Yet, driving a regulation transistor that has high gate capacitance Cg with an amplifier with a limited maximum output current causes an undesirable overshooting phenomena, in certain conditions, at the output of the regulator.
By way of an example, FIGS. 2A, 2B, 2C illustrate a phenomena of voltage overshooting that appears at the output of the voltage regulator of a mobile telephone when the telephone sends data bursts or xe2x80x9cGSM burstsxe2x80x9d at regular intervals (e.g., every 4 milliseconds). FIG. 2A shows the battery voltage Vbat for which the nominal value Vbatnom is 3.5 V. FIG. 2B shows the gate voltage Vg whose value oscillates in the vicinity of a voltage Vgnom equal to Vbatxe2x88x92Vtp when the regulator is stabilized. In this case, this voltage is about 2.8 V if the threshold voltage Vtp of the transistor is 0.7 V. Finally, FIG. 2C shows the output voltage Vout whose rated value Voutnom is 2.8 V when the regulator is stabilized.
At a time t1, the radio circuits of the telephone go into operation to send a burst. The current consumed is very great and the voltage Vbat drops sharply below the rated value Voutnom (FIG. 2A) due to the internal resistance of the battery. The amplifier 2 is unbalanced, the voltage Vg goes to 0 (FIG. 2B), the gate capacitance Cg is entirely discharged, and the transistor 3 is on. The regulator 10 thus works in follower mode, i.e., where the output voltage Vout is substantially equal to the voltage Vbat (FIG. 2C).
At a time t2, the burst is terminated and the power consumed diminishes. The battery voltage Vbat rises again sharply (e.g., in one microsecond) (see FIG. 2A) until it reaches its nominal value Vbatnom. The output voltage Vout follows the voltage Vbat until, at a time t3, it reaches its nominal voltage Voutnom. At this time, the amplifier 2 releases its output from the low state towards the high state and the gate of the transistor 3 is connected to the voltage Vbat by the gate resistance Rg.
This would normally have led to the transistor 3 being immediately turned off. However, as shown in FIG. 2B, the gate voltage Vg increases very slowly due to the high value of the gate resistor Rg, which limits the current delivered, and the high value of the gate capacitance Cg. The output stage of the amplifier 2 is therefore unable to instantaneously charge the gate capacitor Cg and turn off the transistor 3. The transistor 3 continues to be on and the voltage Vout continues to follow the voltage Vbat. As shown in FIG. 2C, a voltage peak OS thus appears at the output of the regulator. This voltage peak cannot dissipate until an instant t4 when the gate voltage Vg crosses the value Vbatxe2x88x92Vtp that turns the transistor 3 off, provided the load Z consumes current.
It is an object of the present invention to limit the effect of overshooting at the output of a voltage regulator in a transient state without the need to modify the structure of a regulation transistor thereof to diminish its gate capacitance.
Another object of the present invention is to limit the effect of overshooting in the transient state without the need to increase the maximum current that can be delivered by the output of the regulation amplifier.
These and other objects, features, and advantages are provided by a voltage regulator including a regulation MOS transistor with low serial resistance and an amplifier whose output drives a gate of the transistor based upon a difference between a reference voltage and a feedback voltage. The regulation MOS transistor has a terminal which receives a supply voltage and another terminal connected to the output of the regulator. The regulator further includes a switch having one of its terminals connected to the gate of the regulation MOS transistor while its other terminal is taken to a potential for turning the regulation transistor off. Also, a switch controller or switch control means monitors the output of the regulator and controls the switch. The switch control means closes the switch when the output voltage of the regulator is higher than a first threshold, where the first threshold is higher than a nominal value of the output voltage.
More specifically, the switch control means are laid out to compare the output voltage of the regulator or a voltage proportional to the output voltage with the reference voltage. The switch control means may include a comparator whose output delivers a signal for closing the switch. The comparator may receive the reference voltage at one input and the output voltage, or a voltage proportional to the output voltage, at another input.
Additionally, the comparator may have a switch-over hysteresis chosen so that the switch is reopened when the output voltage becomes lower than a second threshold. The second threshold may be lower than the first threshold and higher than the nominal value of the output voltage. The regulation transistor may be a PMOS transistor, and the turning-off potential may be the supply voltage.
Also, the amplifier may include an output stage including a gate resistor. A value of the gate resistor is set to be too great for the current flowing through the gate resistor to be capable, on its own, of swiftly turning off the regulation transistor when the supply voltage increases rapidly. Additionally, the switch may be a PMOS transistor having a drain-source resistance in the on state that is far lower than the gate resistance of the output stage of the amplifier.
A mobile telephone according to the invention includes a battery and radio circuits powered by the battery using a voltage regulator as described above.
A method aspect of the invention is for limiting overshooting at an output of a voltage regulator when the supply voltage of the regulator increases rapidly. The regulator includes a regulation MOS transistor with a high gate capacitance, a gate of which is driven by an amplifier delivering a current which, by itself, is insufficient to swiftly turn off the regulation transistor. The method may include connecting a switch between the gate of the regulation transistor and a potential for turning off the regulation transistor. Further, the switch may be closed when the output voltage of the regulator becomes higher than a first threshold, where the first threshold is higher than a nominal value of the output voltage. This temporarily helps the amplifier turn off the regulation transistor.
Additionally, the method may include reopening the switch when the output voltage of the regulator becomes lower than a second threshold. The second threshold may be between the nominal value of the output voltage and the first threshold.